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The Nocturnal Boundary Layer – Balloon Experiments 2005

The Nocturnal Boundary Layer – Balloon Experiments 2005. Caroline Bain - University of Leeds Francoise Guichard (CNRM), Laurent Kergoat (CESBIO), Chris Taylor (CEH), Doug Parker (Leeds) AMMA UK Meeting. Introduction and Motivation.

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The Nocturnal Boundary Layer – Balloon Experiments 2005

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  1. The Nocturnal Boundary Layer – Balloon Experiments 2005 Caroline Bain - University of Leeds Francoise Guichard (CNRM), Laurent Kergoat (CESBIO), Chris Taylor (CEH), Doug Parker (Leeds) AMMA UK Meeting

  2. Introduction and Motivation • Rainfall events in Sahel typified by large mesoscale thunderstorms during the monsoon season (May-September) • The development of these individual systems, although modulated by the large scale vorticity anomalies, is ultimately dependant on surface and boundary layer processes. • In general the south-west monsoon flux is stronger during the night time in the Sahelian region. This is due to the reduction of convective mixing which exerts a drag on the mean winds and weakens the monsoon circulation during the day. • Boundary Layer literature limited by lack of observations • In August 2005, profiles of the nocturnal boundary layer were taken using a tethered balloon

  3. Method • Location: Agoufou fieldsite (15.1N, 1.3W) • Dates: 6th-22nd August 2005 • Night time profiles • Tethered helium filled balloon with simple PTU sonde • Profiles up to 200m in boundary layer • Hourly profiles, readings every 1.5 seconds • Many experimental limitations

  4. Raw Results 14th 6th 11th 18th 7th 15th 20th 12th 21st 13th 17th 9th

  5. 11th/12th Aug

  6. Comparisons to ECMWF 18:22 model – purple obs – pink • Model consistently underestimates speed of nocturnal low level jet • Direction of winds generally good • Theta profile good but some underestimates of inversion in later profiles (too much mixing in model) 00:08 model – dark blue obs – light blue 04:05 model – light green obs – dark green 18:22 model – purple real - pink

  7. 18th/19th Aug

  8. Comparison to ECMWF 19:08 model (18:00) – purple obs - pink 00:02 model – dark blue obs – light blue 05:54 model – light green obs - turquoise • Again, model underestimates wind speeds, though the first diagram differences is due to time lag in comparison of model to observation • Direction of winds good despite large change overnight (though as discussed this is likely to be due to large scale changes which models excel at) • Small bias for model to have cooler temperatures

  9. TRMM/ Meteosat composite Unified Model

  10. Conclusions/ Further Work • Aspects of the observed Nocturnal Boundary Layer are not fully understood • Model underestimates speed of low level jet in comparison to observation case studies • Ongoing research into boundary layer (and soil moisture) and how it effects AEWs • More tethered balloon experiments planned for July/Aug 2006 caroline@env.leeds.ac.uk www.env.leeds.ac.uk/~caroline

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